The present invention relates to a ballast for a discharge lamp such as a high intensity discharge (HID) lamp, and more particularly to an electronic ballast including a DC-DC converter which derives an input DC voltage from a DC voltage source such as a battery to provide a high DC voltage for operating the lamp, and also relates to a method of operating the discharge lamp with the use of the DC-DC converter.
As disclosed in Japanese Patent Early Publication No. 11-260584, a typical electronic ballast for the HID lamp is known to include a DC-DC converter providing an increased DC output, and an inverter which converts the DC output into an AC power for driving the lamp. The DC-DC converter includes a switching element which is driven to repetitively interrupt an input DC voltage to regulate the resulting DC output. Also included in the ballast is a controller which monitors the DC output and varies the duty of the switching element of the DC-DC converter based on the monitored DC output in order to regulate the DC output in a closed-loop control, thereby generating a starting voltage as well as a maintaining voltage to start and keep operating the lamp in accordance with the condition of the lamp.
In order to turn on the lamp, the DC-DC converter is firstly controlled to increase the output DC voltage to a starting voltage which is sufficiently high for initiating the discharge of the lamp after which the lamp is ignited to start the lamp. As soon as the lamp is started, i.e., an arc discharge is established, there is seen a rapid lowering of the output DC voltage associated with a corresponding lamp current lowering. This occurs in such a very short time that the closed-loop control is difficult to follow. As this result, the closed-loop control is likely to delay and rely still upon a high voltage around the starting voltage, thereby causing the DC-DC converter to generate only less amount of the lamp current, although the lamp just after having started will require a more amount of current for successfully proceeding into a stabilized lamp operation. That is, due to the delay in response of the closed-loop control at the very short transition period from the lamp start to the stable lamp operation, the DC-DC converter fails to supply sufficient amount of current to the lamp, which may sometimes cause an unintended extinction of the lamp.
In view of the above problem, the present invention has been achieved to provide an improved ballast for a discharge lamp which is capable of successfully proceeding the lamp start to the stable lamp operation without causing an unintended lamp extinction. The ballast in accordance with the present invention includes a DC-DC converter adapted to receive an input DC voltage from a DC voltage source. The DC-DC converter has a switching element which is driven to repetitively switch the input DC voltage so as to provide a DC output. Included in the ballast is a controller which monitors the DC output and provides a closed-loop control of varying the duty of the switching element based upon the monitored DC output in order to regulate the DC output such that the DC-DC converter provides a starting voltage of initiating a discharge of the discharge lamp as well as an operating voltage of keeping the lamp, operation. The characterizing feature of the present invention resides in that the controller provides an open-loop control which interrupts the closed-loop control to give a start-assisting period in which the switching element is controlled to turn on and off in a predetermined pattern to give DC output of a sufficient level for successfully complementing the lamp start. The start-assisting period is designed to start upon recognition that the DC output satisfies a first reference indicating that the discharging of the lamp is just started and to end upon recognition of a condition indicative of the completion of the starting of the lamp.
Accordingly, the open-loop control can provide sufficient amount of current to the lamp immediately after the starting of the lamp without a delay otherwise associated with the closed-loop control, thereby successfully complementing the lamp start and shifting the lamp to the stable lamp operation.
In a preferred embodiment, the controller is designed to terminate the start-assisting period after the elapse of a predetermined time from the beginning of the start-assisting period. The predetermined time is selected such that the current supplied from the DC-DC converter to the lamp sees no abrupt change when the closed-loop control regains immediately subsequent to the open-loop control.
The ballast may further include an input DC voltage monitor which monitors the input DC voltage supplied from the DC voltage source. The controller operates during the start-assisting period to decrease the on-period of the switching element as the monitored input DC voltage increases. With this arrangement, the DC-DC converter can supply the output of a suitable level irrespective of a possible variation in the input DC voltage, thereby successfully operating the lamp while the available input DC voltage is relatively low, while avoiding an excessive power from being applied to the lamp when the input DC voltage is relatively high, for protection of the lamp and the ballast.
Instead of or in combination with the above scheme of decreasing the on-period, the controller may increase the switching frequency of the switching element as the monitored input DC voltage increases, for the purpose of successfully operating the lamp while protecting the lamp as well as the ballast.
Preferably, the controller is designed to increase the on-period of the switching element with time during the start-assisting period such that a sufficient amount of current can be continuously fed to the lamp in match with the corresponding increase in the lamp voltage within the start-assisting period, thereby successfully completing the starting of the lamp. The on-period may increase in a stepwise manner or continuously.
Further, the ballast may include a lamp-off timer which measures an elapsed time after the discharge lamp is turned off. In this case, the controller is designed to increase the on-period of the switching element as the elapsed time decreases. That is, the shorter the time from the lamp""s turn off to the lamp""s re-start, more amount of current can be fed to the lamp for successfully re-starting the lamp. Speaking differently, the on-period is set larger as the re-start is made sooner after the lamp""s turn off, thereby enabling to supply a sufficient amount of current for restarting the lamp in one hand. And, on the other hand, it is possible to reduce the current when the re-start is made longer after the lamp""s turn off, thereby preventing an excess current from flowing through the lamp and the ballast for protection thereof.
Further, the ballast may include a memory which stores the monitored DC output updated during the start-assisting period so that the controller can rely upon the updated DC output as soon as the closed-loop control regains. With this result, the closed-loop control can use the updated value correctly indicative of the DC output during the start-assisting period for regulating the DC output consistently without suffering from a possible delay in response which would otherwise occur if the DC output is read after the termination of the open-loop control, thereby assuring a smooth transition from the lamp start to the stable lamp operation.
In a further preferred embodiment, the ballast includes a first detector which monitors an output level of the DC output and issues a first status signal indicating that the discharging of the lamp is just started when the output level satisfies the first reference, and a second detector which monitors the output level of the DC output and issues a second status signal indicating that the lamp is already started when the output level satisfies a second reference which is different from the first reference. When both of the first and second status signals are issued, the controller operates to make the open-loop control and the closed-loop control in sequence to regulate the DC output normally. On the other hand, when only the second status signal is issued, the controller responds to disable the open-loop control and makes only the closed-loop control. The closed-loop control in this case operates to generate a relatively low output as compared to that generated by the open-loop control, thereby providing a good basis for determining an abnormal condition of the ballast when the resulting DC output of the relative low level lowers below a low threshold.
When the both of the first and second status signal are issued, the controller responds to provide a first pattern of a power command upon which the closed-loop control relies to regulate the DC output. The first pattern defines the power command decreasing with time to a predetermined level such that the controller can supplement an optimum power to the successfully started lamp in accordance with the first pattern, thereby enabling to proceed the lamp into a subsequent run-up phase with a sufficient light output.
When only the second status signal issued, it may indicate the abnormal condition which results possibly from the ballast becoming partly defective due to short-circuiting or ground fault, for example, yet generating some output due to the presence of a certain circuit impedance. In this condition, the controller provides a second pattern of the power command upon which the closed-loop control relies to regulate the DC output. The second pattern defining the power command which is different and decreases to a predetermined level with time. The second pattern of the power command gives the low power command than the first pattern when the closed-loop control relies at the same time from the start of the closed-loop control. With the use of the second pattern, the DC output is lowered rapidly down below the low threshold, which enables immediate determination of the abnormal condition at the beginning of the closed-loop control not following the open-loop control, thereby protecting the ballast as well as the lamp without causing undue stresses to the circuit components of the ballast. The second pattern can be easily obtained by modifying the first pattern to accelerate the lowering of the power command. Alternatively, the second pattern may be defined as a fixed level not greater than a rated power level.
Preferably, the first reference is a voltage level of the DC output and is lower than a maximum DC voltage applied prior to starting the lamp, and the second reference is a voltage level which is lower than the first reference.
Further, the ballast may include an input current detector which monitors an input current flowing through the switching element. In this connection, the controller has a function of limiting the input current with reference to the monitored input current, and is capable of selecting a first upper limit for limiting the input current therebelow in the open-loop control and selecting a second upper limit which is lower than the first upper limit for limiting the input current therebelow in a rated lamp operation. With the introduction of the first upper limit in the open-loop control, it is made to protect the ballast and the lamp from an excessive current. The first upper limit may decrease with time during the start-assisting period so that a moderate current is supplied later within the start-assisting period to keep the lamp started in accordance with a lamp starting characteristic, yet eliminating unnecessary power. Also, the first upper limit may decrease as the monitored input DC voltage increases so as to provide an adequate amount of current to the lamp for successfully complementing the lamp start irrespective of a variation in the input DC voltage.
These and still other objects and advantageous features of the present invention will become more apparent from the following detailed description of the embodiments when taken in conjunction with the attached drawings